The present invention relates in general to polyurea polyurethanes and in particular to chain extender mixtures for forming high flexural modulus polyurethanes. The invention is also related to reaction injection molding (RIM).
Diethyltoluenediamine and similar aromatic amines have been used, in a relatively pure form, as chain extenders in RIM and other polyurethane processes. The chain extender DETDA is commercially used and comprises a mixture of positional isomers having about 76% 1-methyl-3,5-diethyl-2,4-diaminobenzene and about 24% 1-methyl-3,5-diethyl-2,6-diaminobenzene (generally about 80:20). DETDA and its homolog triethyl m-phenylenediamine are used in RIM processes to provide polyurethanes having acceptable physical properties for molds of relatively limited size. The primary application of these diamines is in the automobile industry where automobile fascia and body panels of intermediate size are prepared.
In order to fill larger molds for large body panels such as doors, fenders, hoods, etc. for automobiles, it is necessary to provide a chain extender which has a somewhat slower gel time, can be handled without breaking at demolding ("green strength"), and has acceptable physical properties, especially heat distortion and impact strength. One of the most critical physical property measurements for large thin body panels and other large molded polyurethane products is flexural modulus measured in pounds per square inch (psi). The flexural modulus is typically measured at -20.degree. F., 75.degree. F. and 158.degree. F.
There exists a need for providing a RIM process and a chain extender composition which provides a reaction injection mixture that has an appropriate gel time for filling large molds, has suitable properties at demolding, and has acceptable physical properties including flexural modulus.
In the past, attempts have been made to increase the flexural modulus of polyurethane parts by increasing the proportion of diamine that goes into the polyol resin mixture for reaction with a polyisocyanate. However, this has only resulted in polyurethane formed products which are brittle at demold and cannot be handled to be postcured at elevated temperatures. Furthermore, even if the brittle parts are postcured without handling the finished polyurethane products still retain some brittleness and are unacceptable when an excessive portion of the diamine is used in the reaction mixture.